Method of making adherent score-resistant coating for metals

ABSTRACT

A bearing element having an adherent score-resistant coating on its surface comprising a carbonized layer of an organic resin in which the layer is carbonized by ion implantation. The ion implantation is controlled to not only provide graphite-like and diamond-like structures in the carbonized coating but to also produce chemical bonding between carbon atoms of the resultant coating and atoms of the bearing surface.

RELATED PATENT APPLICATIONS

This patent application is a division of prior patent application Ser.No. 674,239 filed on Nov. 23, 1984, now U.S. Pat. No. 4,554,208.

FIELD OF THE INVENTION

This invention relates to an adherent score-resistant coating formetals. More specifically it involves an improved antifriction coatingfor bearing surfaces.

BACKGROUND OF THE INVENTION

When metallurgically compatible metal surfaces are rubbed together, themetal of one surface can metallurgically, i.e. chemically, bond to themetal of the other surface. Such action, for example, is the basis forfriction welding and cold welding. The bond, of course, can be as strongas the metal itself. If one attempts to slide the two surfaces on oneanother after even minute surface areas are so bonded, the slidingaction can cause metal to be pulled away from one of the surfaces. Whenthe metal is pulled away from the surface, that surface is consideredgalled. The mating surface then has a corresponding bump on it that canproduce further galling, or even scuffing and scoring. We refer to suchaction generally as wear. In any event, sliding movement of the twosurfaces will cause friction between them to increase. When friction isextreme, bearing surfaces can even seize together. The rate at whichthis will occur will, of course, vary with pressure applied during thesliding, surface compositions, surface roughness, etc.

To prevent such problems, one approach is to provide an antifrictionand/or score-resistant coating on one or both of the mating metalsurfaces. We have found a coating that is highly effective as anantifriction or score-resistant coating on metal surfaces, particularlyiron-based alloy surfaces in contact with metallurgically compatiblemetal surfaces. The coating is hard, tenacious and distinctive.Moreover, it is formed by an unusual process.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved article ofmanufacture that includes a bearing surface having an adherent layerthereon formed of an organic resin carbonized by ion implantation.

Another object of the invention is to provide a bearing element havingan antifriction coating with graphite-like and diamond-like crystallitestherein and the coating chemically bonded to the bearing surface.

Another object of the invention is to provide a method for forming anadherent score-resistant coating on metals, especially iron basedalloys, by a distinctive ion implantation technique.

Our invention comprehends preparing a bearing surface by coating a metalsubstrate surface with a film of a sulfur-free organic resin, andimplanting ions into the coating by ion bombardment. This ionimplantation is initially done at a first energy selected to bothcarbonize the coating and reduce its thickness at least about 50%.Thereafter, ion implantation is continued at a lesser energy to targetthe ions to produce maximum implantation damage to chemical bonds at theinterface between the coating and the substrate surface. Implantation iscontinued at the lesser energy for a sufficient duration to completelydecompose the organic resin into substantially pure carbon, and to bondthe resulting carbon atoms to themselves and to the substrate surface.In the resultant product, we have observed evidence of graphite-like anddiamond-like crystalline forms in the bulk of the coating and chemicalcompounds of carbon and the metal in the coating-substrate surfaceinterfacial zones. In addition, when silicon ions are the implantationion species, silicon carbide forms appear.

Other objects, features and advantages of this invention will becomemore apparent from the following description of preferred examplesthereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of carbonizing a sulfur-free organic resin to provide a hard,adherent coating is not new. It has been used to make high densityintegrated circuit masks for exposure to deep ultra-violet light. U.S.Pat. No. 4,410,611 to B. A. MacIver claims the method of carbonizingneeded to make such a mask. U.S. Pat. No. 4,321,317 to B. A. MacIverclaims a unique photolithographic process using such a mask to make highdensity integrated circuits. In substance, MacIver found that if aphotoresist was first ion implanted to reduce its thickness and then ionimplanted to bond it in place, a tough, adherent coating was producedthat was substantially opaque to deep ultraviolet light. It thuspermitted one to avoid an undesirable and costly photoetching step whenmaking integrated circuit photomasks.

In this invention, we recognize that the MacIver ion implantationprocess produces a film that also has utility as a score-resistantcoating on metal surfaces, particularly surfaces that are in slidingcontact with one another, i.e. bearing surfaces. We find that theMacIver process produces a coating of substantially pure carbon thatappears to have graphite-like and diamond-like microcrystals therein, tomake the coating not only low in friction but hard. We refer to such acoating as carbonized. Morever, we believe we can use the MacIverprocess to actually chemically bond the carbonized coating to theunderlying metal surface, to make a coating that is extremely adherent.The result is a tenacious score-resistant coating.

On the other hand, we have discovered that this coating is only usefulon a surface of metallurgically, or chemically, compatible metal. Bythat we mean a metal which will form chemical compounds with carbon. Webelieve that covalent carbon-metal bonds are established incoating-substrate interfacial zones that provide the extreme adherenceof the coating. This chemical, or metallurgical bonding, is anespecially important attribute, that makes the coating function as wellas it does in this invention. Further, we have found that to beeffective as a score-resistant coating, the bearing surface must bequite smooth, as for example 8 microinches roughness average or less forcoating thicknesses less than about 0.2 micrometer. A surface roughnessaverage of less than about 4 microinches (0.1 micrometer), preferably2-3 microinches (0.05-0.075 micrometer), is generally desired for thebearing surface. In substance, we have found that if a surface is of theright metal and if it is smooth enough, the MacIver integrated circuitprocess can be used to make a score-resistant coating.

In a specific example of this invention, a 0.6 micrometer thick coatingof polymethyl methacrylate (PMMA) positive-type electron resist wasapplied to the standard General Motors Corporation production cast ironworking face of a valve lifter for an internal combustion engine. Theelectron resist is the same sulfur-free organic resin MacIver describesusing in the aforementioned integrated circuit patents. We choose to useit for convenience, as will hereinafter be more fully explained. Theresist coating was applied in essentially the same way MacIver describesfor integrated circuit manufacturing processes. A drop of thephotoresist was placed on the foot of the valve lifter and the valvelifter then axially spun to remove excess photoresist. With its workingface up and horizontal, the valve lifter was baked in air at about 80°C. for about 30 minutes, to drive off excess resist solvent. The coatingworking face of the valve lifter body was then given a uniform ionimplantation across its entire working face, i.e. foot, using ²⁸ Si⁺ atan energy of approximately 200 keV and a flux of 3 microamperes persquare centimeter in a dosage of 3×10¹⁵ silicon ions per squarecentimeter. We prefer silicon ions because they provide an incidentalbenefit, as will become more apparent. This implantation was at anenergy sufficient to cause the silicon ions to completely penetrate theresist coating. In fact, we prefer that the ions penetrate into themetal surface far enough at the very start of this implantation to causemaximum damage to occur at the coating-metal interface right at theoutset. We will explain more on this point later.

The foregoing implantation caused the organic resin coating to shrink inthickness from its original 0.6 micrometer thickness to about 0.16micrometer. The coating was apparently carbonized to a considerableextent and bonded to some extent. However, it apparently was not bondedwell enough to the underlying metal face to be scratch-resistant.

The entire coated working face of the valve lifter was then given asecond uniform implantation with ²⁸ Si⁺. However, this time theimplantation energy was at 150 keV at a flux of 3 micrometers per squarecentimeter and in a dosage of 5×10¹⁵ silicon ions per square centimeter.The organic resin coating shrank in thickness only slightly, to about0.15 micrometer, and became highly scratch-resistant.

The static and kinetic coefficients of friction of the valve lifter'scoated working face were then measured under both dry and lubricatedconditions. They were measured by placing the coated working face of thevalve lifter, which had a surface roughness average of about 5microinches (0.125 micrometer), on a horizontal flat ground steelsurface, which also had a surface roughness average of about 5microinches (0.125 micrometer). To determine the static coefficient offriction the lateral force needed to overcome the lifter's surfacecontact forces and cause the lifter to start to slide across the steelsurface was measured. It was first measured without an oil film on thesteel surface, and then with one. The same test was made with a similarvalve lifter that did not have the score-resistant coating on itsworking face. In the latter sample, the working face of the similarvalve lifter had a surface roughness average of about 4 microinches (0.1micrometer). The uncoated working face had a static coefficient offriction of 0.22 in the dry test and 0.27 in the lubricated test. Thecoated working face had a static coefficient of friction of 0.15 in thedry test and 0.19 in the lubricated test. Accordingly, the percentreduction in friction coefficient was 32% dry and 30% lubricated.

The kinetic coefficient of friction was analogously measured on both ofthe aforementioned samples under both dry and lubricated conditions. Thekinetic coefficient of friction was measured by measuring the lateralforce necessary to keep the valve lifter sliding across the horizontalsteel surface after it has been given a push to get started. As in thepreceding test, the valve lifter rested on the flat ground steel surfacewith its working face contacting the steel plate. The kineticcoefficient of friction for the coated working face was 0.15 when thesteel surface was dry, and 0.16 when it had an oil film on it. Thecoefficient of friction for the valve lifter with the uncoated workingface was 0.18 when the steel surface was dry, and 0.17 when it had theoil film on it. This represented a 17% reduction in kinetic friction dryand 6% reduction in kinetic friction lubricated.

This invention was also tested using an unetched SAE 390 aluminum alloyslider in a typical type of friction and score test apparatus. Theapparatus used in this test is described by John C. Bierlein in hispaper entitled "Measurement of Plain Bearing Material Score Resistance"that was published in the Proceedings of the National Conference onPower Transmission, Triple Engineering Show & Conference, volume VI,pages 51-57, Chicago, Ill., Nov. 13, 1979. In this test the slider isplaced in contact with the axial face of a flat steel disk rotating at aconstant velocity. The steel disk had a surface roughness average ofabout 4 microinches (0.1 micrometer). Kerosene was used as a fluidlubricant between the slider and the disk face during the test. In thistest the force pressing the slider and disk face together is increasedlinearly with time, while maintaining the predetermined constant diskrotation. The torque needed to maintain the slider in fixed position isused as a measure of friction between the slider and disk face. When thedisk face was coated with the score-resistant coating of this invention,no significant increase in friction was observed even at the end of anormal test force cycle for this apparatus. On the other hand, when thesame test was performed with an uncoated disk, the test failedcatastrophically after only about 15% of the test force had beenapplied. By catastrophic, we mean that the slider seized on the disk,and the force needed to maintain the slider in fixed position increaseddrastically. It is of interest to note that very few prior antifrictioncoatings have exceeded the limits of this test apparatus. On the otherhand, the foregoing results have been repeatedly obtained with otherdisks coated in accordance with this invention.

As previously mentioned, the integrated circuit resist PMMA was used toproduce the organic resin coating that is implanted in this invention.We do believe that any methacrylate resist can be used but that theinvention is not limited to resists of that type. For example, webelieve this invention can be used on any sulfur-free resist, as forexample polymethyl isopropenyl ketone (PMIK), Shipley's AZ-1350, TokyoOka Kogyo's OMR-83 or OSR, hexafluorobutyl methacrylate (HFBM), methylmethacrylate (MMA) or poly (alpha-methyl styrene) prepared by plasmapolymerization (alpha-MS). For the reasons mentioned in the followingparagraph we believe that many organic resins can be used. The onlyexception identified so far is that the resin should not contain anyappreciable amount of sulfur. In essence, we want a resin that willleave a carbon deposit when decomposed by ion implantation.

We further believe that this invention is applicable to any sulfur-freeorganic resin that can be applied in thicknesses of 0.5-1.0 micrometers.At least 0.5 micrometer generally is needed in order to insure oneobtains a continuous film. More than about 1.5 micrometers cannot beused because of the ion implantation energy required to penetrate it.Such equipment is simply not commercially available. On the other hand,we would prefer to employ as large a thickness as we can to get maximumwear resistance. In general this means using an initial coatingthickness of about 1.0 micrometer, to insure the coating will becompletely penetrated in the first implantation. On the other hand, ifion implantation equipment of a power greater than about 200-250 keVbecomes available, we would, of course, want to increase the thicknessof the film. In fact, we would prefer to use the maximum thickness thatcan be penetrated in the first implantation by the available ionimplantation equipment.

Coating uniformity is important in targeting the ions. The organic resincoating should be applied highly uniformly, to permit uniform ionimplantation maximum damage along the entire resin-metal interfacialarea. This provides most uniform bonding along the entire interface.Hence, we believe that any organic resin can be used that will form auniform thickness film of 0.5-1.5 micrometers and will carbonize underthe implantation we described. As a matter of convenience, the mostreadily available materials that we know of that will produce such filmsreliably are used in integrated circuit manufacturing resists. That iswhy we prefer using them.

From the foregoing it should be apparent that we prefer to use thelargest practical energy we can in the first implantation, to permit usto use the thickest possible starting thickness in the organic coating.Energies of at least about 180 keV are contemplated for the firstimplantation. In any event it must be of sufficient power to provide animplantation range of the selected ion species that is as far as thecoating is thick. By range we mean the average depth of penetration ofthe implanted ions. It appears important even in this firstimplantation, that the average implanted ion at least reach thecoating-metal interface, or adhesion in the resultant coating will notbe maximum. Maximum adhesion is insured if the range is targeted toproduce maximum damage, i.e. ion mixing which promotes the formation ofnew chemical bonds, precisely at the coating-metal interface. It wouldseem that this targeting is more important to the second implantation.However, it appears of some importance in the first implantation too,perhaps to provide some initial bonding before the coating starts todecompose. We prefer that the implantation be conducted at asufficiently rapid rate, i.e. flux, of at least about 2 microamperes persquare centimeter and preferably higher.

Both the first and second implantation coincidentally must not only beat high energies and fluxes but also at dosages of at least about 1×10¹⁵-5×10¹⁶ ions per square centimeter for initial resin thicknesses of0.5-1.0 micrometers. The first implantation should have a dosage highenough to reduce the thickness of the organic resin to at least about50%, and preferably to about 25%, of its original thickness. Then thesecond implantation will cause little, if any, additional shrinkage. Ifthe second implantation does not cause any significant shrinkage, ionscan remain targeted for maximum damage at the coating-metal interfaceduring the entire second implantation, not just a part of it. Insummary, the dosage needed in the first implantation is related tostarting resin film thickness. Why a dosage of at least about 1×10¹⁵atoms per square centimeter also seems to be needed to get satisfactorycoating-metal bonding is not clearly understood. In any event, dosagesin excess of 5×10¹⁶ atoms per square centimeter do not appear to provideadded benefits in either the first or second implantation, and may evenbe objectionable.

It has been mentioned that the energy of both the first and secondimplantations should be targeted to provide maximum damage at the resincoating-metal interface. By maximum damage, we mean the maximum damageto lattice structures, i.e. atomic displacement or interfacial atommixing at the coating-metal interface. Such damage is typified byvacancies being produced in a crystal lattice or atoms being moved intointerstitial positions in the crystal lattice. We believe that when thisaction occurs from impact of the bombarding ions, their momentum istransferred to atoms of a material lattice which they hit, permittingthe formation of new bonding relationships. The first implantation notonly decomposes the oganic resin into substantially pure carbon (plusthe implanted ion species) but also apparently forms newcarbon-to-carbon relationships. For example, x-ray diffraction studiesshow evidence of both graphite-like and diamond-like crystalline formsin the resultant coating when dosages of at least 1×10¹⁵ atoms persquare centimeter are used in both the first and second implantations.The coating has both amorphous and polycrystalline graphite structuresthat incorporate diamond-like structures with particle sizes beingtypically less than 50 nanometers. Presumably, both the graphite-likeand the diamond-like crystalline microstructures are incidentally formedduring both the first and second implantations.

We believe that coating attributes are further enhanced by using siliconas the implant ion species. The silicon is metallurgically compatiblewith the carbon and produces microstructures of silicon carbide in situ.Silicon is also metallurgically compatible with metals such as iron,aluminum and copper.

It should be mentioned that maximum ion implantation damage is producedat about 60-80% of the ion implantation projected range. As used herein,ion implantation projected range means the average depth of penetrationof the ion species in the coating and the metal. We believe that theplace where maximum damage occurs will be the place where most newchemical relationships will form. In other words, by targeting the zoneof maximum damage to be at the coating-metal interface, we maximizeformation of the carbon-to-metal bonds, and thus maximize adhesion ofthe resultant coating to the metal surface.

As indicated above, the second implantation is performed at an energylevel targeted to produce maximum damage at the coating-metal interfaceto promote chemical bonding between the carbon produced in the coatingand metal surface atoms. This carbon-metal bonding produces aninterjacent zone of carbon-metal compounds which tenaciously bond thecoating to the underlying metal surface. Dosages in excess of about5×10¹⁵ atoms per square centimeter do not provide any enhanced bonding.In fact, in some instances it would appear that dosages in excess ofthis amount may produce poorer results, perhaps by a sputtering etchingeffect on the carbon atoms.

It should be understood that the transfer of energy provided by the ionbombardment of this invention is not critically dependent upon anyparticular ion species. On the other hand, one should select an ionspecies that is metallurgically compatible with the carbon-metal systeminvolved. By that we mean the ion does not disrupt desired chemicalrelationships, and preferably enhances them, as for example by formingchemical compounds. For example, silicon ions can form compounds withboth carbon and iron. Ordinarily one would want to use an ion speciesthat has a low molecular weight and still be compatible in the systeminvolved. In some instances an ion species would be consideredmetallurgically compatible because it is normally a gas, such as argon,neon, etc., and forms volatile compounds or no compounds, so that theseions can be annealed away. Other ion species that will carbonize anorganic resin coating and high adhesion on metals such as iron-basedalloys are phosphorus ions, argon ions, boron ions, and boron difluorideions. I expect that nickel ions, nitrogen ions, and carbon ions wouldalso be suitable, depending upon the metal or alloy.

We believe this coating can be formed on many metals, such as iron-basedalloys and aluminum-based alloys. By iron-based alloys, we mean anymetal containing over 50% by weight iron. By aluminum-based alloys, wemean a metal containing over 50% by weight aluminum. When used onaluminum alloys, however, we believe that the primary use would be inlight unit load applications. Under heavy unit loads, the aluminumsurface supporting the coating may tend to deform, which probablyreduces effectiveness of the coating.

We believe that our coating wears little. On the other hand, it is sothin, i.e. about 0.1-0.2 micrometer, that even a little wear can be aproblem if concentrated in location. We have found that this coatingworks quite well as an antifriction or score-resistant coating when itssupporting metal surface is quite smooth. By that we mean that the metalsurface has a roughness average of less than about 8 microinches. Infact, we prefer that the metal surface have a roughness average nogreater than about 4 microinches (0.1 micrometer). Best results havebeen obtained when surfaces were no rougher than about 2-3 microinchesaverage (0.05-0.075 micrometer). In retrospect, it appears that when ourcoating is used on rough metal surfaces, it tends to wear away at highpoints on the rough surface. This action causes the underlying metalsurface to lose the protection afforded by the coating. Accordingly, ifour coating could be produced in thicknesses greater than about 0.1-0.2micrometer, presumably the surface supporting it could probably berougher. For example, for a coating thickness of 0.4 micrometer, asurface roughness average of about 10-12 microinches (0.25-0.3micrometer) might be acceptable.

It should also be mentioned that we consider this process as one inwhich targeting of ion implantation maximum damage is maintained at thecoating-metal surface interface by reducing the ion implantation energyin a second step. It should be recognized that one can effectivelyaccomplish the same result in a series of steps or that one cancontinuously reduce implantation energy so as to continuously maintainmaximum damage at the coating-metal surface interface.

While this invention has been described in connection with producing anadherent antifriction coating on the working face of an automotive valvelifter body, it can be used in any application where there is a slidingcontact between two mating surfaces, provided that at least the coatedsurface, and preferably both surfaces, is sufficiently smooth. Thiscoating can be used on piston and other seal rings, cam surfaces, spoolvalves, axles, and pivot points. It is of particular interest as acoating on iron-based alloy surfaces to reduce friction when thosesurfaces are moved against ductile metal surfaces, such as those ofaluminum-based alloys and copper-based alloys. It may even prove to beeffective when the other metal surface is also an iron-based alloy. Thisinvention is expected to be useful in such applications, particularlywhen the coating is on an iron-based alloy moving in contact withaluminum-based alloy surfaces.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of making anadherent coating on a bearing surface of an iron based alloy memberhaving a surface roughness average of less than about 8 microinches,which coating exhibits high lubricity and score resistance when insliding contact with an aluminum based alloy surface, the methodcomprising:applying a coating about 0.5-1.0 micrometer thick of asulfur-free, methacrylate-type of resist to a bearing surface of an ironbased alloy member; initially implanting ions into the coated surface atan energy greater than about 180 keV at a rate greater than about 2microamperes per square centimeter and at a dosage of about 1×10¹⁵-1×10¹⁶ ions per square centimeter, wherein the ions are of a substanceof gaseous atmospheric pressure or which can chemically bond to carbonor iron, and have sufficient energy to completely penetrate the resistcoating, and are implanted in sufficient dosage to reduce coatingthickness to about one-half to one-fourth of the initial coatingthickness; and subsequently implanting ions into the coated surface atan energy of about 100-150 keV at a rate of at least about 2microamperes per square centimeter at a dosage of about 1×10¹⁵ -1×10¹⁶ions per square centimeter, wherein the energy is tailored to providemaximum damage to existing chemical bonds to occur and thereby allowcarbon-to-metal bonds to form at the interface between the bearingsurface and the coating, effective to produce a coating containinggraphite-like and diamond-like crystalline microstructures and initiallybond the coating to the iron based alloy bearing surface.